The present invention relates to a method for reducing crazing in a plastics material, in particular a transparent plastics material.
When transparent plastics materials are used for windows, roofing, signs or light fittings maintenance of their original optical clarity is important. Unfortunately, under the influence of environmental factors such as light, heat and moisture many plastics materials suffer from crazing. Crazing is a phenomenon where microvoids form in the body of the materials. These microvoids may not cause a significant deterioration in mechanical strength of the article, but they do reflect/refract light and decrease the clarity of the article. Ultimately, crazing decreases the strength of the article and can lead to failure.
As crazing is a visually obvious deterioration in the material, it gives an impression of poor quality or lack of maintenance which is particularly objectionable in applications where visual clarity is desired. Such applications include the windows of transport vehicles, roofing sheets, light fittings or signs. Many signs are made of transparent materials with the graphic material applied to the underside to be viewed through the material. Crazing in vehicle windows interferes with the vision of the occupants decreasing their enjoyment of the journey and may even pose a real safety hazard. Crazing is particularly objectionable in aircraft windows and frequently causes the replacement, at a great expense, of windows which are otherwise sound and serviceable. As a consequence of the crazing problem, the use of glass windows is being considered for aircraft despite the weight penalty that this would impose.
The cause of crazing is unknown and may be manifold. It is thought that one cause is the diffusion of small molecules such as water or surfactants into the material which decreases the attractive forces between polymer chains and allows movement of molecules under internal or external stress thus forming microvoids.
According to the present invention there is provided a method for reducing crazing in a plastics material which comprises the steps of:
(1) cleaning the surface of the material; and
(2) exposing the cleaned surface to plasma of a monomer vapour so as to produce a substantially non-oxidising plasma polymer coating on the surface.
The method of the present invention may be used to reduce crazing in a wide variety of plastics materials, such as, for example, acrylics, styrenes, polycarbonates, polyesters or polyurethanes. The plastics material may be an article which is preferably in the form of a laminate or sheet. The method will have particular value when applied to transparent plastics material where visual clarity is important. Examples of transparent materials include acrylic or polycarbonate sheets as used for the windows of transport vehicles such as aircraft, boats, trains and motor vehicles, signs or for architectural uses such as in roofing, glazing sheets and light fittings.
The material may be cleaned in step (1) by any method which leaves the surface substantially free of any contamination capable of interfering with the adhesion of the plasma polymer coating. A preferred method of cleaning the surface is to subject the material to a low pressure plasma of an inert gas such as argon, neon, or nitrogen. Another preferred method of cleaning the surface involves subjecting the material to a low pressure plasma of an oxidising gas such as air or oxygen. Water vapour is also a suitable oxidising gas for this purpose. These cleaning methods may be advantageously carried out in the same apparatus which is used in step (2) of the method.
The monomer used in step (2) may be any saturated or unsaturated organic compound capable of producing a coating of a substantially non-oxidising polymer containing organic groups.
Suitable saturated monomers include siloxanes, fluorinated compounds, lower hydrocarbons, lower alcohols, lower alkylamines and mixtures thereof The term xe2x80x9clowerxe2x80x9d as used herein refers to monomers containing 1 to 12 carbon atoms.
Suitable unsaturated monomers include acrylic esters, methacrylic esters, vinyl esters, vinyl aromatics, unsaturated or polyunsaturated hydrocarbons and mixtures thereof. Examples of these monomers include methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, methyl methacylate, ethyl methacrylate, n-butyl methacrylate, 2-ethylhexyl methacrylate, vinyl acetate, styrene, p-chloromethylstyrene, 2-vinylpyridine, 4-vinylpyridine, N-vinylpyrrolidone, vinyl halides of the formula CH2xe2x95x90CHX wherein X in Cl or F, vinylidene halides of the formula CH2xe2x95x90CX2 wherein X is independently Cl or F, vinyl ethers of the formula CH2xe2x95x90CHOR wherein R is alkyl, and allyl derivatives such as allyl ethers, allyl carbonates or diallyl carbonates.
Plasma polymers from some of these monomer classes typically undergo extensive oxidation on aging (Gegennbach et al, J Polymer Sci, Part A Polymer Chemistry, 32, 1399-1414 (1994); Gegennbach et al, Surface Interface Analysis, in press 1996). In those cases it is necessary to carefully adjust the plasma deposition parameters until minimal oxidation following ageing in the air is obtained. While substantial oxidation can occur in plasma polymers without affecting their structural integrity, minimal oxidation lessens the danger of adverse changes to the surface or mechanical properties of a plasma polymer as it ages. As used herein, the term xe2x80x9csubstantially non-oxidising polymerxe2x80x9d refers to materials which show such minimal oxidation.
It has been found that for windows made from acrylic polymers such as those used in aircraft, the substantially non-oxidising polymer coating is preferably hydrophobic. Siloxanes or perfluorinated compounds are particularly suitable monomers for producing hydrophobic coatings provided that the resulting polymer contains some organic groups. Examples of such monomers include hexamethyldisiloxane, vinyltrimethoxysilane, perfluorocyclohexane and tetrafluoroethylene.
For plastics materials where the crazing is caused by exposure to hydrophobic molecules such as petroleum products, a hydrophilic coating may be more suitable in which case monomers such as alcohols or alkylamines may be used. Preferred examples of such monomers include methanol, ethanol and the various isomers of propanol or butanol.
The plasma polymer coatings produced by the method of this invention are usually highly crosslinked and hence stable. They may also be abrasion resistant.
Many of the materials to which the method of the present invention can be applied are subject to varying stresses in service and move or flex slightly as a consequence. Accordingly, there is a need to match the mechanical compliance of the coating with that of the material. The present invention achieves this by ensuring that the plasma polymer coating applied in step (2) is thin and adheres well to the material so that it moves with the material without itself cracking or crazing. It is preferred that the plasma polymer coating has a thickness of about 2 to about 500 nm, more preferably about 5 to about 50 nm.
The prior art methods produce thicker coatings which are unable to follow the movement of the material and crack and/or delaminate.
The method of the present invention may be carried out in any suitable apparatus for performing plasma polymerization such as that described in AU 654131. AU 654131 describes a process for plasma coating polymeric materials in a vapor of an amide monomer so as to provide a coating suitable for the growth of cells on biomedical implants to be administered into the human body. Preferably, low pressure plasma polymerization is employed in which the pressure is about 0.11 to 1.0 torr, preferably about 0.5 to 1.0 torr.
The present invention also provides a craze resistant article comprising a plastics material having a thin coating of a substantially non-oxidizing plasma polymer containing organic groups.